Currently, in many old cities of the United States, the sewer network systems consist of combined sewer systems, separated sanitary sewer systems, and storm water sewer systems. A combined sewer is designed to collect the combined sewage of domestic and industrial wastewater, and storm water runoff in the same pipe. A separated sanitary sewer is designed to collect domestic and industrial wastewater. However, a storm water sewer is used to collect storm water runoff only. During the periods of dry weather or light rainfall, the urban sewer network transports all of the collected combined sewage from combined sewer systems and wastewater from sanitary sewer systems to a treatment plant to receive full treatment before discharging to a nearby water body. However, during heavy storm events, the quantity (flow rate and volume) of the collected sewage and storm water runoff in a combined sewer system may surpass the designed capacity of the sewer system or the treatment plant. When this situation occurs, the excess flows will inevitably overflow to a nearby water body. These overflows are known as combined sewer overflows (CSOs). With increasing frequency super storms in recent years, for example, Hurricanes Katrina, Irene and Sandy, have caused flood and overflow of urban sewer systems and devastating damages to both properties and humans.
Combined sewer overflows can cause serious water pollution problem. The deposition of sewage solids during dry weather in combined sewer systems has long been recognized as a major source of receiving water pollution. One of the underlying reasons for considerable sewage solids deposition is the combined sewer hydraulic design. Dry-weather flow velocities are typically inadequate to maintain settleable solids in suspension, and a substantial amount of solids tend to accumulate in the sewer systems. During rain storms, the accumulated solids may re-suspend, and overflow to receiving waters due to the limited hydraulic capacity of the interceptor. Suspended solids concentrations of several thousand parts per million are not uncommon for CSOs. This can produce shock loadings detrimental to receiving water. Development of a means to control or remove sedimentary deposits is required to prevent their undesirable effects.
The control of CSOs employing structural measures such as sewer separation, storage and treatment has been used in a number of major cities in the United States. Nationwide application of these techniques for the control of CSOs would require expenditures over 100 billion dollars. New strategies are needed to reduce these costs to tolerable limits. Sewer sediment flushing can significantly reduce overall costs when integrated with other upstream management practices and downstream storage tanks. Engineered sewer sediment flushing systems are low-cost control alternatives which can be viewed as an added measure for structural control and treatment. In some cases, the CSO storage-sedimentation facility may be more cost effective for controlling suspended solids and associated pollutants; however, it requires efficient flushing systems for removing tank bottom sediments.
Concern over sewer flushing can be dated back to the Romans. In the U.S., early historical efforts for sewer sediment cleaning occurred in Syracuse, N.Y. at the turn of the century. The method for sewer cleaning is to create a flushing wave to scour and transport the deposited sediments to a storage sump by rapidly adding external water or by quickly opening a flushing gate.
Currently significant work has been invested to achieve a cost effective means to purge the sediment deposited in combined sewers, CSO storage tanks, and storm water conveyance systems via a variety of flushing techniques. Existing flushing technologies include Hydrass®, Hydroself®, Biogest®, Huber Power Flush®. All of these flushing systems require either an external sources of water and/or energy or complex control mechanism. Therefore, United States Environmental Protection Agency (USEPA) further developed a sediment flushing system as disclosed in U.S. Pat. No. 6,655,402 to C.-Y. Fan, which is hereby incorporated by reference. The USEPA's system can be installed either in a CSO storage tank or in a combined sewer. Notably, the invention creates effective hydraulic waves without the use of an outside energy source, but uses water from the storm event itself. However, a major limitation of USEPA's flushing system is that when the water level outside the flushing tank drops slowly (near the opening of the vacuum break pipe), the flushing wave is weak. This is caused by a direct relationship between the completed vacuum break time and the speed at which water is released from the flushing tank.
Therefore, to effectively remove sewer solids from urban drainage systems between storms, a flushing system with high efficiency and cost effectiveness, yet without need of external sources of water and energy and complex control instrumentation, is still urgently needed.